One of known methods of nonlinear compensation for a power amplifier is a predistortion method. The predistortion method adds a distortion component in advance to the input signal so as to cancel the distortion component that occurs in a power amplifier. In this specification the distortion component that is added by the predistortion method will hereinafter be referred to as a compensation signal. An ideal compensation signal is set to be equal in level to but 180° out of phase with the distortion component that is actually created by the power amplifier. The amount of compensation for distortion by the predistortion method depends on accuracies of the amplitude and phase of the compensation signal. For example, when the input-output characteristics of the power amplifier are formulated by a power series model, a compensation signal keeps amplitude and phase deviations of each odd-order distortion component within ±0.28 dB and ±1.8°, respectively for the attainment of a 30 dB compensation.
Generally speaking, as the operating point of the power amplifier approaches saturation power, the power added efficiency (hereinafter referred to as efficiency) increases. However, operation of the power amplifier around the saturation power region causes an increase in the distortion component. To attain a desired amount of attenuation of distortion outside a signal band (an adjacent-channel leakage power ratio or the like), higher distortion compensation is required than in the case of operating the power amplifier in a low-efficiency region.
However, the nonlinear characteristic of the power amplifier in the around the saturation power is so complex that it is not easy to generate a compensation signal which provides the above-mentioned amplitude and phase deviations for each odd-order distortion component. One of the factors for complicating the nonlinear characteristic is so-called memory effects in the nonlinear characteristic itself (for example, W. Bosch and G. Gatti, “Measurement and Simulation of Memory Effects in Predistortion Linearizer,” IEEE Trans. Microwave Theory Tech., Vol. 37, pp. 1885–1890, December 1989, hereinafter referred to as non-patent document 1). Such memory effects leads to convolution of the distortion component by a band pass filter with certain characteristics, thereby adding a frequency characteristic to the distortion component that occurs in the power amplifier. On this account, to attain predetermined distortion compensation over a certain frequency band, the compensation signal needs to be set so that the amplitude and phase of each odd-order distortion component stay within predetermined deviations throughout the frequency band. Another factor for complication of the nonlinear characteristic is the generation of not only 3rd-order but also 5th- or higher-order distortion components at the output of the power amplifier. To achieve high distortion compensation, the compensation signal is required to have such frequency characteristics as compensate for the frequency characteristics attributable to the memory effect, and a higher-order compensation signal needs to be generated.
With a conventional predistortion linearizer based on power series, hereinafter referred to as a power series predistortion linearizer, (for example, Japanese Patent Application Kokai Publication 2003-229727, hereinafter referred to as patent document 1, and Nojima and Okamoto, “Analysis and Compensation of TWT Nonlinearities Based on Complex Power Series Representation,” Journal of IEICE (B), Vol. J64-B, No. 12, pp. 1449–1456, hereinafter referred to as non-patent document 2), the input signal is exponentiated and adjusted its amplitude and phase to generate a compensation signal. However, no desired frequency characteristics can be imparted to the compensation signal, and for this reason it is impossible to achieve high distortion compensation.
Predistortion methods of compensation for frequency characteristics are proposed, for instance, in Japanese Patent Application Kokai Publication 2002-64340, hereinafter referred to as patent document 2, and Japanese Patent Application Kokai Publication 2002-57533, hereinafter referred to as patent document 3. In particular, in patent document 2 the output from the distortion generator is divided into high- and low-frequency components of the fundamental signal, and the both frequency components are adjusted in amplitude and in phase independently of each other to provide the compensation signal with frequency characteristics. In patent document 3 amplitude-frequency characteristics adjusting circuit composed of a band pass filter and a vector adjuster is disposed behind a distortion generator to add frequency characteristics to the compensation signal.
However, patent documents 2 and 3 do not clearly disclose how to obtain the desired frequency characteristics for the compensation signal. Further, these patent documents are suggestive of the configuration for compensating for 3rd- and higher-order distortion components, but they are silent about a concrete method of compensating for the higher-order distortion components. The compensation for high-order distortions presents such a problem as mentioned below.
When a certain order compensation signal is added by the predistortion linearizer, the added compensation signal leads to generation of a new distortion component at the output of the power amplifier. The newly developed distortion component affects the compensation effects on distortion components of other orders. As a result, there is possibility that the compensation for distortions of the other orders does not become optimum. Hence, it is necessary to take into account such interdependence between the compensation effects on the distortion components of different orders.